I'm trying to build an Instrumentation amplifier that would do a 10uV -> 10mV amplification. The motivation is to measure uA currents on a small enough shunt resistor (1-10Ohm).
For a proof of concept, I've built a circuit like the one below with a distinction that I'm powering it with two 9V batteries and am using LM2902N(http://www.kynix.com/uploadfiles/pdf9675/LM2902N.pdf) as the op amps and Rg is somewhat different. It has much less that the required 1000x gain.
However, when measuring the results for different input voltages, the gain seems to vary:
I was expecting it to be constant.
And here are my questions:
What is a reasonable place to look for the causes of the variations?
- Cheap resistors?
- Inadequate op amp?
- Breadboard prototyping?
- 2x9V batteries as power supply?